PTC thermistors are commonly used in conjunction with refrigeration and air-conditioner compressors. These thermistors serve to separate, by fractional distillation, lubricating oil and compressor fluid in the crank case of the compressor. To accomplish such separation, it is necessary to insure that an efficient thermal relationship is established between the PTC thermistor and the receptacle in which it is housed. Several approaches have been devised to accomplsih an efficient heat exchange relationship. For example, compressor manufacturers have applied silicone sealing grease around the PTC during installation, whereby the PTC thermistor will conduct heat directly to the casing of the compressor. It is also known to mold the PTC thermistor in a thermally conductive silicone rubber, and when the heater is energized in the casing, the rubber will expand to provide a tight fit. Each of these approaches is somewhat messy and not overly efficient in heat transfer.
PTC thermistors are especially useful in situations in which there is a need for a heater that regulates its own temperature. When electrical current is directed through the PTC thermistor, it tends to heat and display increasing resistivity during heating so that current in the thermistor is reduced whereby its rate of heat generation is decreased. With PTC thermistors, when the rate of heat generation reaches equilibrium with the rate of the heat dissipation, the thermistor's temperature stabilizes and limits the resistor current to a predetermined level. The initial room temperature resistivity of a PTC material and the rate of change of resistivity with temperature are characteristic of the material, and the materials used in such thermistors are commonly chosen to display a sharp anomalous increase in resistivity at a particular temperature, thereby to stabilize heating of the thermistor at about that temperature while also reducing resistor current to a very low level at the stabilizing temperature.
PTC thermistors have been in use for many years and heaters utilizing such thermistors offer several operating advantages over conventional resistance heating elements in the heating of various fluids. They can be made in a flat shape, formed generally of a doped barium titinate ceramic which has a sharp positive temperature coefficient of resistance. The PTC thermistor are designed such that below the critical or anomaly temperature, the resistance of the ceramic that forms them remains at a low value and is essentially constant. When a particular temperature is reached, a crystalline phase change takes place in the ceramic and this change in the ceramic structure is accompanied by a sharp increase in the resistance at the crystalline grain boundaries. With the unique temperature characteristics of the PTCs, they are extremely valuable in providing heat at precise temperatures whereby to vaporize certain fluids and not vaporize others, thereby effectively separating the fluids into their components. The temperature at which the crystalline change takes place can be adjusted in the PTC thermistor manufacturing process through the use of appropriate chemical dopents and can be varied between about -50.degree. C. and 300.degree. C. When energized with a suitable current by applying voltage to the opposite sides of the device, the PTC thermistor rapidly heats up to a predetermined operating temperature and then "locks in" at that temperature. This rapid heating is due to the initial low resistance of the PTC ceramic which results in an internal high power of the heater. The lock-in is due to the abrupt increase in the resistance which causes generated power to reduce until it equals dissipated power. At this point, thermal equilibrium is achieved and the PTC thermistor self-regulates itself at that temperature, and when used with compressors or air conditioners to separate fluids into fractions, only the compressor fluid will be vaporized, while leaving the lubricating oil in a liquid state. The high rate of heat transfer is accomplished by the specific construction of the heater of the present invention which enables heat to be readily transferred from the PTC thermistor to the fluids in the vessel in which the heating device is housed.